Along with miniaturization of semiconductor devices, it has become more difficult to form micropatterns by lithography utilizing ultraviolet light. Therefore, lithography technologies using X-rays, electron beams, ion beams, etc. have been proposed, researched, and developed.
As previously proposed electron beam transfer type lithography techniques, PREVAIL (projection exposure with variable axis immersion lenses) developed jointly by IBM and Nikon, SCALPEL (scattering with angular limitation in projection electron-beam lithography) developed by Lucent Technologies etc., and LEEPL (low energy electron-beam proximity projection lithography) developed jointly by LEEPL Corporation, Tokyo Seimitsu Co., Ltd., and Sony can be mentioned.
For PREVAIL and SCALPEL, a high energy electron beam of an acceleration voltage at about 100 kV is used. In the case of PREVAIL and SCALPEL, an electron beam passing through part of a mask is focused on a resist by a reduction projection system of a scale factor of usually 4 to transfer the patterns.
For LEEPL, a low energy electron beam of an acceleration voltage at about 2 kV is used (T. Utsumi, Low-Energy E-Beam Proximity Lithography (LEEPL) Is the Simplest the Best? Jpn. J. Appl. Phys. Vol. 38 (1999) pp. 7046-7051). In the case of LEEPL, the electron beam passes through holes provided in a mask to transfer patterns on a resist at the same scale.
LEEPL has an advantage in simplifying the configuration of the electron lens barrel compared with PREVAIL and SCALPEL. Also, generally, the higher the acceleration voltage of the electrons, the less the scattering of the electrons in the resist and the less probability of reaction of the electrons and the resist. Therefore, in lithography utilizing a high energy electron beam, a more sensitive resist is required. As opposed to this, in LEEPL, since the energy of the electron beam is low, the resist can be used at a high sensitivity and a high productivity can be realized.
FIG. 1 is a schematic view of LEEPL exposure. As shown in FIG. 1, a stencil mask 101 used for LEEPL has a thin film (membrane) 102. Holes 103 corresponding to the patterns are formed in the membrane 102. The membrane 102 is a part of a membrane formation layer 102a. The membrane formation layer 102a around the membrane 102 is formed with a support frame (frame) 104 for reinforcing the mechanical strength of the stencil mask 101.
The stencil mask 101 is arranged in proximity to the surface of a wafer 105. The wafer 105 is coated with a resist 106. When scanning the stencil mask 101 by an electron beam 107, the electron beam 107 passes through only the portions of the holes 103 so the patterns are transferred on the resist 106. Since LEEPL is same scale exposure, it was necessary in conventional LEEPL to make the size of the membrane 102 several mm to several 10 mm square or equal to the size of a LSI chip on which the patterns are transferred.
FIG. 2 is an enlarged perspective view of part of the membrane 102 of FIG. 1. As shown in FIG. 2, the membrane 102 is formed with holes 103 corresponding to the micropatterns. For etching the membrane 102 to form the holes 103 with a high precision, generally a ratio of the membrane thickness to the diameter of the holes 103 (aspect ratio) must be 10 or less, preferably 5 or less. Therefore, when forming the holes 103 for the patterns having a line width of for example 50 nm in a stencil mask for production of a device of the 0.10 μm or later generation, it is necessary to make the membrane thickness 500 nm or less.
The thinner the membrane thickness, the more precisely the holes 103 can be formed. However, a membrane 102 formed thinly easily flexes. If the membrane flexes, the transferred patterns may distort or the transferred patterns may become offset in position. Therefore, the membrane 102 is formed so that tensile stress occurs inside. The larger the area of the membrane 102, the greater the internal stress required for flattening the membrane 102.
FIG. 3 shows the change of deflection and internal stress of a membrane depending on the membrane area. Here, the membrane is made a rectangular shape with four fixed sides. The length of one side is indicated on an abscissa of FIG. 3. The deflection shows the deflection at the center of the membrane due to gravity, while the stress shows the stress occurring at the center of the membrane. FIG. 3 shows an example of calculation for a silicon nitride film having a thickness of 200 nm assuming a Young's modulus of 300 GPa.
Flattening the membrane requires an internal stress able to cancel out the stress at the center. In the example of FIG. 3, when the membrane size becomes larger than 10 mm square, the stress at the center will exceed 10 MPa. Therefore, an internal tensile stress of 10 MPa or more is required at the membrane.
Although it is possible to increase the internal stress to fabricate the membrane, if forming holes in a membrane in the state of a large internal stress, the internal stress is released at the hole parts. Therefore, as shown in for example FIG. 2, when forming a plurality of holes of different shapes from each other unevenly in the membrane or forming holes having large diameters, offset or distortion of the patterns easily occurs around the holes.
Separate from the above problems, in the case of a stencil mask, there is the restriction that formation of specific patterns requires use of a complementary mask. A membrane mask comprised, without holes, of a substrate formed with a light-blocking film (or bodies for scattering a charged particle beam) may be formed topologically with donut-shaped interconnection patterns without problem. As opposed to this, in the case of a stencil mask, since all of the parts except the holes must be connected, when forming donut-shaped interconnection patterns, it is necessary to divide the patterns among a plurality of masks and to perform multiple exposure using these masks.
Alternatively, when forming holes corresponding to long line-shaped patterns, anisotropic distortion occurs in the pattern shapes due to the influence of the internal stress so the line width will not become even or stress will concentrate at corners of the patterns and the membrane will easily break. Therefore, long line-shaped patterns are also sometimes divided into a plurality of rectangles and continuous patterns are transferred by multiple exposure.
In the above way, when using a stencil mask for electron beam transfer type lithography, multiple exposure using a plurality of masks is assumed and the patterns have to be aligned with a high accuracy.
Further, in recent semiconductor devices, the number of interconnection layers forming the multilayer interconnections has been increasing. Securing alignment accuracy of the patterns between layers has been becoming increasingly difficult.